Optical spatial filters have been of great importance for optical information processing systems. They are described at length in the Introduction to Fourier Optics by J. W. Goodman, McGraw Hill, 1968, pages 141-197. In these systems the synthesis of the desired linear systems response is typically accomplished by inserting in the optical beam path precisely positioned spatial filters with carefully designed spatially varying transmissivities. Changing these filters, however, requires a significant period of time which results in measurements taken with different filters being a relatively slow procedure.
There has been considerable research on optical materials whose spatial varying transmissive properties can be changed in real time. These include photochromic materials such as those described by R. Exelby and R. Grinter in Chem Reviews G5 (2), page 247 (1965) and liquid crystals such as described by G. W. Gray in "Molecular Structure and Properties of Liquid Crystals" Academic, New York 1962. Changeable spatial filters could be constructed from these materials. However, such filters would have several disadvantages. One disadvantage is that the rate at which filter functions could be changed would be of the order of milliseconds at best. A second disadvantage would be that every time a particular filtering function would be desired, that function would have to be completely rewritten, since the medium would have no memory of previously used filter functions.